The present invention relates generally to protective relaying in a power distribution system, and more particularly to a method for use in connection with a protective relay or like device to provide an accurate reach measurement method.
In a power distribution system, electrical transmission lines and power generation equipment must be protected against faults and consequent short circuits. Otherwise, such faults and short circuits can cause a collapse of the system, equipment damage, and/or personal injury. Accordingly, and as shown in FIG. 1, a typical power system employs one or more protective relays to monitor system conditions on a protected transmission line, to sense faults and short circuits on such protected line, and to appropriately isolate such faults and short circuits from the remainder of the power system by tripping pre-positioned circuit breakers on such protected line.
As seen, a typical power system can be connected over hundreds of miles and include multiple power generators (generator S, generator R) at different locations. Transmission lines (the main horizontal lines in FIG. 1) distribute power from the generators to secondary lines or buses (the main vertical lines in FIG. 1), and such buses eventually lead to power loads. Importantly, relays and circuit breakers arc appropriately positioned to perform the isolating function described above.
A modern protective relay typically measures and records voltage and current waveforms on a corresponding protected line, and employs a microprocessor and/or digital signal processor (DSP) to process the recorded waveforms. As used herein, the term xe2x80x98transmission linexe2x80x99 includes any type of electrical conductor, such as high power conductors, feeders, etc. Based on the processed waveforms, the protective relay can then decide whether to trip an associated relay, thereby isolating a portion of the power system.
In particular, and referring now to FIG. 1A, it is seen that a typical protective relay 10 samples voltage and current waveforms VA, VB, VC, IA, IB, IC from each phase (A-C) of a three phase line 12. Of course, greater or lesser numbers of phases in a line may be sampled. The sampled waveforms are stored in a memory 14 and are then retrieved and appropriately operated on by a processor or DSP 16 to produce estimated impedances and phasors. As should be understood, such impedances and phasors are employed to determine whether a fault condition exists, and if so to estimate fault location.
Based on the estimated impedances and phasors, then, the relay 10 may decide that an associated circuit breaker 18 should be tripped to isolate a portion of the line 12 from a fault condition or from other detected phenomena, and issue such a command over a xe2x80x98TRIPxe2x80x99 output (xe2x80x98TRIP 1xe2x80x99 in FIG. 1A) that is received as an input to the circuit breaker 18. The relay 10 may then reset the circuit breaker after the relay 10 senses that the fault has been cleared, or after otherwise being ordered to do so, by issuing such a command over a xe2x80x98RESETxe2x80x99 output (xe2x80x98RESET 1xe2x80x99 in FIG. 1A) that is received as an input to the circuit breaker 18.
Notably, the relay 10 may control several circuit breakers 18 (only one being shown in FIG. 1A), hence the xe2x80x98TRIP 2xe2x80x99 and xe2x80x98RESET 2xe2x80x99 outputs. Additionally, the circuit breakers 18 may be set up to control one or more specific phases of the line 12, rather than all of the phases of the line 12. Owing to the relatively large distances over which a power system can extend, the distance between a relay 10 and one or more of its associated circuit breakers 18 can be substantial. As a result, the outputs from the relay 10 may be received by the circuit breaker(s) 18 by way of any reasonable transmission method, including hard wire line, radio transmission, optical link, satellite link, and the like.
As seen in FIGS. 1 and 2, transmission lines are oftentimes series-compensated by series capacitance in the form of one or more capacitors or banks of capacitor installations (a representative series capacitor CAP is shown). Benefits obtained thereby include increased power transfer capability, improved system stability, reduced system losses, improved voltage regulation, and better power flow regulation. However, such installation of series capacitance introduces challenges to protection systems for both the series-compensated line and lines adjacent thereto.
Typically, and as best seen in FIG. 2, installed series capacitance includes a metal oxide varistor (MOV) or other non-linear protection device in parallel with the series capacitance (CAP), which limits the voltage across the capacitance in a pre-defined maimer. Additionally, a bypass breaker or bypass switch (SW) is installed in parallel with the series capacitance, which closes at some point following operation of the MOV. Typically, and as seen, the breaker is controlled by a protective relay 10 via an appropriate BYPASS output (FIG. 1A). Conduction through the MOV and the closing of such breaker introduce transients in the system as the impedance seen by the protective relay is altered. The quick response of the MOV, the breaker, and the spark gap (SG) installed in parallel with the series capacitance removes or reduces the capacitance and limits the impact of the transient.
Protection of a power distribution system with one or more series compensated lines is considered to be one of the most difficult tasks both for relay designers and utility engineers. A protective relay should be designed to have a high level of security and dependability. A utility engineer should be able to set the protection properly. However, protection settings depend on prevailing system conditions and system configuration, and both may change significantly if series capacitors are present in the system. In particular, such changes result from the fact that series compensation elements installed within a power system introduce harmonics and non-linearities in such system, arising from the aforementioned MOV, bypass switch, spark gap, and other elements. A protective relay or like device must therefore have an accurate reach-measure scheme to take proper action, especially in view of the changes resulting from installed series capacitance and its related elements.
In the present invention, an accurate reach-measurement scheme improves the reach-measurement of impedance relays and the fault location estimation using local information only. The improvement is accomplished by numerically solving the ordinary differential equation that describes the series installed capacitance installation. The scheme is simple and accurate and requires only local voltage and current at the bus. Furthermore, the scheme easily adapts to different series installed capacitance installations and operation of the installed capacitance protection, and is independent of surrounding power system elements. Existing numerical relays can easily incorporate the new reach-measurement scheme in their protection functions so that such improvement is achieved on a minimal cost basis.
In particular, in the present invention, a reach-measurement method is used in connection with a series-compensated line of a power system. The series-compensated line includes an installed series capacitance having a bus side and a line side, and a non-linear protection device parallel to the installed series capacitance. The series-compensated line has a line current, a bus side voltage, and a line side voltage. The series capacitance and the non-linear protection device have a capacitance voltage thereacross equal to the bus side voltage minus the line side voltage.
In the method, a number (n) of line current samples are measured, where such samples are representative of values of a line current waveform at successive instants of time on the series-compensated line. Capacitance voltage values are computed based on the measured line current samples in accordance with an equation which takes into account the non-linear protection device parallel to the installed series capacitance. A prescribed power system function is then performed based on the computed capacitance voltage values.